clippy: Use `unwrap_or_default()`

Clippy was unhappy:

```text
error: use of `.unwrap_or_else(..)` to construct default value
   --> sdk/src/signer/keypair.rs:171:27
    |
171 |     let derivation_path = derivation_path.unwrap_or_else(DerivationPath::default);
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `derivation_path.unwrap_or_default()`
    |
    = note: `-D clippy::unwrap-or-else-default` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_else_default
```

sdk/src/signer/keypair.rs

@@ -168,7 +168,7 @@ pub fn keypair_from_seed_and_derivation_path(
     seed: &[u8],
     derivation_path: Option<DerivationPath>,
 ) -> Result<Keypair, Box<dyn error::Error>> {
-    let derivation_path = derivation_path.unwrap_or_else(DerivationPath::default);
+    let derivation_path = derivation_path.unwrap_or_default();
     bip32_derived_keypair(seed, derivation_path).map_err(|err| err.to_string().into())
 }